This invention relates to a system that determines the phase and amplitude of an occulted radio signal using open loop tracking.
Radio occultation remote sensing of the atmosphere, termed xe2x80x9cradio occultationxe2x80x9d herein, includes the measurement of the phase and amplitude of a radio signal where the atmosphere or ionosphere of a planetary body is positioned between a transmitter and a receiver. The phase measurements can be very accurate and they contain information about the refractivity of the atmosphere or ionosphere and about the planetary surface.
Radio occultation has been applied to the earth""s atmosphere with the use of Global Positioning System (GPS) transmitters. As shown in FIG. 1, the earth 100, the earth""s atmosphere 102, and the earth""s ionosphere 103 are positioned between an occulted transmitter system, such as Global Positioning System satellite 104, and a receiver system, such as Low Earth Orbiting (LEO) satellite 106, that is equipped with a radio occultation receiver. The Global Positioning System satellite 104 orbits the earth 100 at an approximate altitude of 20,200 km. The Low Earth Orbiting satellite 106 orbits the earth 100 at an approximate altitude of 300-1,000 km. The radio occultation receiver can also be placed on any airborne platform, such as an aircraft or balloon, or can be ground-based. The Global Positioning System satellite 104 transmits a radio signal 108 to the Low Earth Orbiting satellite 106. As the radio signal 108 traverses the earth""s atmosphere 102 and ionosphere 103, the phase and amplitude of the radio signal 108 can be measured by the Low Earth Orbiting satellite 106 and then inverted to compute atmospheric parameters for weather and climate applications. Another application of radio occultation uses ground-based receivers 110 to obtain the phase and amplitude data.
The received signal can be used to compute signal bending angles. The bending angles calculated from the radio occultation signals are further used for determining the atmospheric refractivity, or for direct assimilation into Numerical Weather Prediction (NWP) models. The determined refractivity is further used for determining pressure, temperature, and humidity or for direct assimilation into Numerical Weather Prediction (NWP) models. The phase of ground-received radio occultation signals can be used to extract information about atmospheric moisture or for direct assimilation into Numerical Weather Prediction (NWP) models. The bending angles calculated from the ground-received radio occultation signals can be used to correct radar observations at low elevation angles.
In both of these applications of radio occultation, the receiver systems 106, 110 have used digital Phase-Locked Loop (PLL) signal processing to extract the phase and amplitude from the radio occultation signal 108. In Phase-Locked Loop signal processing, the phase and amplitude are extracted in real time, after down conversion of the input radio occultation signal 108, using a phase model that is recurrently updated by extrapolation of the previously extracted phase. The feedback between the phase model and the input radio occultation signal 108 makes the Phase Locked Loop signal processing an optimal tracking technique for single tone signals that are corrupted with noise. On the other hand, the feedback makes tracking of the received radio occultation signal 108 that has complicated dynamics, such as multiple tone signals, an unstable process and may result in errors in the output phase. Also, the use of feedback results in an inability of the system to track signals, i.e., in the loss of lock, under the conditions of low signal-to-noise ratio (SNR) and complicated signal structure.
To obtain an accurate calculation of the bending angles of the radio signal occulted by the earth""s atmosphere, multiple tones, contained in the radio occultation signals, must be resolved. For space-received radio occultation signals, multipath propagation in the moist troposphere section of the earth""s atmosphere results in strong fluctuations of both phase and amplitude. In particular, random phase accelerations are much larger than those that can be tracked by the Phase-Locked Loop signal processing in a Low Earth Orbiting (LEO) satellite 106 receiver. For ground-received radio occultation signals, the multipath is caused primarily by signals reflected from the earth""s surface. Reflections from the sea surface cause periodic deep fades in the amplitude of the radio occultation signals. In both applications, the Phase-Locked Loop signal processing is not capable of tracking radio occultation signals without a corruption or loss of lock. Also, the Phase-Locked Loop signal processing is not capable of tracking a threshold of the rising occultations, as it needs time to maintain lock on the signal. In both applications, the radio occultation data are most valuable at low altitudes (elevations).
Open-Loop (OL) tracking has been used in radio occultation studies of planetary atmospheres to complement the Phase Locked Loop signal processing. Open-Loop tracking is basically the raw sampling of the down converted complex signal. In Open-Loop tracking there is no feedback between the phase model used for the down conversion and the received signal. The advantage of Open-Loop tracking is that it is not susceptible to the complicated structure of the input signal and the input signal is never lost. One disadvantage of Open-Loop tracking is the low Signal-to-Noise Ratio (SNR) due to aliasing of the noise into the sampling frequency domain. This disadvantage is especially perceptible when tracking radio occultation signals with low gain antennas, such as those used on micro-satellites.
The above-described problems are solved and a technical advance achieved by the present system for determining the phase and amplitude of a radio occultation signal that modifies the traditional Open Loop tracking process to maximize the signal-to-noise ratio, minimize the sampling rate, and also preserve the structure of the radio occultation signals. A radio occultation system includes a transmitter system, a receiver system, and a post-processing system. The receiver system receives the radio signal that is transmitted by the transmitter system through the earth""s atmosphere where it is occulted and down converts the received radio occultation signal in real-time to generate a down converted signal based on a phase model. The receiver system then low pass filters the down converted signal and samples its in-phase and quadrature components. The post-processing system receives the in-phase and quadrature components of the down converted and low pass filtered signal, determines the mean residual frequency and performs additional down conversion of the sampled signal to eliminate aliasing. The post-processing system then re-samples the additionally down converted signal at a higher sampling rate and calculates the accumulated phase and amplitude from the re-sampled signal. The present system for determining the phase and amplitude of a radio occultation signal therefore obtains a signal-to-noise ratio that is comparable to Phase Locked Loop signal processing, by using Open-Loop tracking.